1. Field of Endeavor
The present invention refers to the field of gas turbines, to a method for operating a gas turbine, to an application of the method in a combined cycle power plant with integrated gasification, and also to a combined cycle power plant with integrated gasification useful for carrying out the method.
2. Brief Description of the Related Art
A combined cycle power plant with integrated gasification (integrated gasification combined cycle, IGCC) is normally operated with a synthetic gas (syngas or Mbtu gas), which is produced by gasification of coal, biomass, or other fuels (see, for example, U.S. Pat. No. 5,901,547). In order to improve the availability of the power plant and to ensure continuous power generation, an auxiliary operation either with natural gas or with a liquid fuel is normally necessary during the maintenance times of the gasification unit.
Such IGCC power plants are customarily started with an auxiliary fuel such as natural gas or oil, and then switched over to syngas after the gasification unit has been put into operation. The switching-over of fuel (fuel switchover, FSWO) from the auxiliary fuel to syngas is a very complex changeover process which is generally associated with load fluctuation, pulsations, and the risk of flashback. Furthermore, the risk of a trip of the gas turbine during the FSWO is extremely high.
In specific cases it is possible to individually switch over the fuel for each of the plurality of burner groups in a combustion chamber of the gas turbine, and so to reduce the extent of the load fluctuations or the risk of a trip of the gas turbine. This group-wise operation can also be used for a mixed combustion, where individual burner groups are operated with different fuel compositions, such as syngas/oil or syngas/natural gas.
The main problem with this previously described type of operation is an increased risk of flashback or of pulsations due to different flame shapes and pressure drop coefficients of burners in the same combustion chamber in the case of different fuels.
On the other hand, gas turbines have been known for quite some time (see U.S. Pat. No. 5,577,378 and A. Eroglu et al., Entwicklung des sequentiellen Verbrennungssystems für die Gasturbinenfamilie GT24/GT26, ABB Technik 4/1998, S. 4-16 (1998) (Development of the Sequential Combustion System for the GT24/GT26 Gas Turbine Family, ABB Technology 4/1998, p. 4-16 (1998))), which operate with reheating by a second combustion chamber. Unlike single-stage combustion, the sequential combustion with reheating by a second combustion chamber offers the possibility of operating each of the combustion chambers with different fuels. Such gas turbines with sequential combustion therefore enable new types of starting, operation, and mixed combustion in an IGCC power plant.
U.S. Pat. No. 5,689,948 discloses a gas turbine installation with sequential combustion, in which the two combustion chambers are basically operated with the same gaseous fuel. Reference is certainly made to the addition of oil as “auxiliary fuel” in the second combustion chamber in order to ensure the self-ignition.
U.S. Pat. No. 6,167,706 discloses a gas turbine installation with only one combustion chamber (105 in FIG. 8). A second external combustion chamber (106) only indirectly yields heat to compressed air, but does not act directly on a further turbine. The simultaneous use of different fuels for the two combustion chambers is not explicitly mentioned but only a series of possible fuels summarily listed.
Publication DE-C-947 843 discloses a gas turbine installation with a plurality of combustion chambers which, however, are all operated with the same gas from a pressure gasifier, which gas results either directly as pressurized gas or as sluice gas.
U.S. Pat. No. 6,116,016 discloses a gas turbine installation (FIG. 6) with two (sequential) combustion chambers, of which the first is operated with oil or natural gas, and the second operated with coal-derived gas.
Publication EP-A1-0 723 068 disclosed a gas turbine installation with sequential combustion, in which the two combustion chambers are operated with gaseous fuels of different H2 content, which fuels result during a reforming process.